Tannin staining of coatings on wood is an undesirable spontaneous process resulting from migration of tannins from the wood, and which is especially acute in the case of aqueous white coatings or aqueous clear coats. Particularly, such staining is a problem in coatings applied on hardwoods such as oak, noted for its high water-soluble tannin content. On oak and other woods such as redwood or red cedar, such staining is observed as dark brown discoloration of coatings or clear coats, which develops cumulatively, causes aesthetic degradation and often limits the service life of wood coatings. Tannin staining of solvent-based opaque coatings or clear coats is notably less prevalent, even on wood substrates of high staining potential.
Under federal and state legislative pressure, intended to minimize volatile chemical emissions, significant efforts of the paint and coating industries are currently invested into development of high performance water-based wood coatings. To enhance the tannin stain-inhibitive properties of such coatings is one of the challenges of the related development work. It is informative to refer briefly to some elements of wood chemistry, the mechanism of the tannin staining process and to the available preventive technologies.
Wood consists mainly of cellulosic and ligninous materials. Diverse wood species also contain variable amounts of extractables, some of which are water-soluble and colored and consequently accountable for staining. It is well known, however, that not all water-soluble colored extractables are tannins, and not all tannins are water soluble. The content of such extractables varies widely among different wood species. The quantitative distribution of the same is known to be variable even among distinct anatomical regions of tree specimens.
The chemical composition, structure and chemistry of colored extractable species is typically complex and not always well known. In addition to tannins, of which redwood, for example, contains approximately 4-12%, other water soluble, colored organic products of complex structure, such as quinones, flavonoids and tropolones are also present in various wood species. It will be interesting to observe that thujic acid, an intensely colored member of the latter family is known to be present in western red cedar, a popular but highly staining wood. Wood is a highly porous material, characterized by a remarkably high average value of capillary specific surface area of about 2000 cm.sup.2 /g. The cellular walls, and, to limited extent, the cellular cavities or lumens are microscopic storage sites for some soluble extractables, which are concentrated predominantly in heartwood, the physiologically inactive anatomical region of living trees.
Wood is both porous and hygroscopic. It absorbs water vapor up to approximately 30% of the original moisture content by weight, (the fiber saturation point achievable at 100% relative humidity of air) or desorbs moisture in a dynamic equilibrium with the moisture content of the surrounding air. The above absorption process (up to 30% weight increase) is associated with dimensional instability known as swelling and shrinking of wood, considered to be one of the major causes of the physical degradation of the wood coatings.
It will be observed, however, that when exposed to condensing humidity conditions, water content of wood could reach 200% by weight, without additional swelling, because swelling reaches its peak at the fiber saturation point.
Notably, polar organic solvents such as aliphatic alcohols, amines, glycols and derivatives of the same, common components of aqueous or solvent-based paint formulations, are also absorbed by wood, as the result of their ability to form hydrogen bonds with cellulosic --OH groups.
An indirect consequence of porosity and hygroscopicity of wood is tannin staining, the spontaneous and complex process which results in a loss of decorative value, or, if extensive, limitation of the useful service life of wood coatings.
During staining several processes occur concurrently, in a complex and dynamic equilibrium. Practically, it starts at the moment of application, as the paint film's water content penetrates at a relatively high rate and rehydrates the substrate's surface layer. Consequently, the cellular walls, typically collapsed in dry wood materials, are restored to their expanded, porous structure, characteristic of green wood. As a result, water-soluble staining species normally stored in cellular walls or lumen cavities of wood materials, are solubilized, mobilized by absorbed water and thus made available for migration. Consequently, mobilized staining species, driven by concentration gradients will diffuse spontaneously through substrate-coating interfaces into the coatings and toward coating-air interfaces, where cumulative staining occurs.
It is informative to observe, that the high liquid volume ratio of fresh coating applications provides a system open for diffusion, and it is primarily accountable for the high staining rates observable during the film drying period. As a result, aqueous coatings freshly applied on highly staining wood substrates are considerably discolored at the end of the film drying period, typically in about 1 to 2 hours. Concurrently with the above described diffusional processes and depending on the surrounding conditions, water also evaporates, at a variable rate, through the coating-air interface. Because of both processes, fresh coating applications are substantially depleted of water in short time, by the end of the above mentioned film drying period.
Although latex particles coalesce simultaneously and reach an advanced degree of curing by the end of the water evaporation or film drying stage, the completion of the film formation or curing process generally requires a considerably longer time.
It will be noted, however, that cured coatings (inclusive of solvent-based systems) are perfectly water-permeable, and consequently, coated wood substrates interact spontaneously with water, in typical fashion, as previously described. As a result, tannin staining proceeds spontaneously and cumulatively all during the service life of wood coatings, according to the above described mechanism.
The aesthetically acceptable limit of the extent of stain discoloration of white coatings is a matter of subjective judgment. In practice, however, it is reasonable to consider that limit's numeric values (expressed in CIELab) at dE=20-25, measured comparatively against a coating formulation's intrinsic color values on non-staining substrates.
It is important to observe, that if no other destructive processes interfere, high staining rates, which result in substantial discoloration in a short period of time, may limit the useful service life of wood coatings. Notably, however, staining rates of white opaque coatings and clear coats on wood substrates vary widely, a function of service conditions and pertinent formulation parameters. Under normal service conditions, it is the surrounding atmosphere's relative humidity content (R.H.), which determines the staining system's (wood substrate and cured coating) water content and consequently, limits the rate of the related discoloration process. There are experimental data to indicate, that discoloration of latex coatings on redwood proceeds at significant rates at R.H. values close to 100%, which correspond to the fiber saturation point of the substrate. On the other hand, at R.H. &lt;30% the same values are negligible.
Alternatively, under condensing humidity conditions, wood substrates are (at least intermittently) saturated with water and consequently, staining rates will be diffusionally limited, controlled essentially by the related coating's physical structure and more specifically, by the permeability or porosity of the same. Dependent on intrinsic porosity and composition, wood coatings display semi-permeable behavior with respect to staining processes. Without obstructing substantially the rate of water absorption by the substrates, they limit however, the in situ diffusion rate of dissolved colored substances and, consequently, determine the pertinent rate of self-discoloration.
As is well known, organic coatings' porosity is variable to a considerable extent, a function of certain formulation parameters. For example, the inverse proportionality between coatings' PVC values and resultant porosity is common knowledge in the art of paint manufacturing.
At any given PVC value, however, the porosity of an organic coating depends significantly on the resin component's film-forming characteristics, as well. In that respect, it is generally well known, that solvent-based resins typically form coatings of low porosity, which also display good stain inhibition capacity. Conversely, aqueous acrylic latex-based paint formulations are known to form comparatively porous coatings, which, as indicated above, are characterized by relatively poor stain inhibitive characteristics.
Notably, however, as is common knowledge in the art, the film-forming characteristics of aqueous latices vary significantly as a function of quality parameters, such as polymer structure, average latex particle size, Tg temperature, film-forming temperature and coalescent solvent, among others. In general, in the development of latex-based paints, all of the above-mentioned formulation parameters as well as price are optimized for most cost-effective stain-inhibitive capability of the resultant coating. Additionally, to as above-mentioned, however, there are specific procedures available to enhance effectively the staining inhibitive capability of latex-based wood coatings. The employment of stain inhibitor pigments or "tannin blockers" constitute such procedures used in the art. See, for example, my U.S. Pat. No. 5,529,811. Pigment grade stain inhibitors are believed to function by adsorbing or chemosorbing tannin species in situ in the coatings, preferably as light-colored compounds. A severe limitation of such inhibitors, however, is their incompatibility with aqueous clear coats and stains intended for application to wood.
A distinct approach to enhancing the stain inhibitive capacity of aqueous wood coatings is disclosed in U.S. Pat. No. 5,320,872 by T. E. McNeel et al., who recommends the addition of diverse complexing agents to the pertinent paint formulations. It is interesting to observe, that complexing agents do not directly interact with staining species such as tannins, but rather prevent the formation of intensely colored tannates, and specifically Fe-tannates, by complexing the available transition metal ions in situ of coatings.
Considering the above presented examples, it is apparent, that heretofore known procedures achieve inhibition of tannin staining as a result of specific chemical interactions, which take place in situ with wood coatings, between staining species (or alternatively, heavy metal cations such as Fe(III), as noted above) and functional constituents (or additives) to the coatings, such as stain inhibitor pigments.
It is also apparent that the possibility of enhancing the stain inhibitive capacity specifically of latex based coatings, by reducing the porosity or permeability of the same and thus, by minimizing the in situ diffusion rates of staining species, thus minimizing the rate of observable staining has not been recognized.